I tend to animate to kill time, or if I'm bored, I often experiment with different programs and see what I like.
Pivot Animator I think is VERY good. It could use some improvment, like making the browsers stop detecting it as a virus, but overall it's pretty great.
I think Flipnote is perfect, very easy to use. I have a Sudomemo account, check that out here.
Below are some animations I am most proud of. Yeah I know, they suck. (They are sorted from oldest to newest.)